In order to design longitudinal studies the within-subject variability of the measured parameters must be known. However, information is limited about the within-subject variability of parameters of coagulant and fibrinolytic activity in postmenopausal women. This shortage restricts the range of studies that can be designed to assess the impact of hormone replacement therapy (HRT) on coagulation and fibrinolysis in this population. Therefore, we measured a battery of relevant factors in 34 healthy postmenopausal women on 4 separate occasions over 3 months. We found that certain parameters (eg., plasminogen activator inhibitor-1) varied greatly over time within individuals [eg., coefficient of variation (CV) 55%], whereas others (eg., thrombin-antithrombin complex concentrations) varied much less (eg., CV = 11%). The information we accumulated will be used to determine how frequently different parameters must be measured to detect significant biological changes that are temporally related to an intervention, such as the administration of HRT.